This invention concerns compression/decompression networks. Specifically, the invention concerns modifying the characteristics of compression/decompression tables to facilitate data throughput and memory efficiency.
Codeword resolution is important to accurately reconstruct coded data. Fixed-length quantization lookup tables allow a compression network to efficiently quantize and dequantize data using minimal processing. Quantization tables using a greater number of bits to represent output codewords allow better resolution than tables using a fewer number of bits to represent output codewords. However, more bits require more memory for storing the data after quantization and a greater bandwidth to transfer the same data. Quantization tables are lossy in that, for a given quantization level, they compress several input data points of approximately the same value to one output value. During reconstruction, the same dequantized value represents all data points within the resolution of a particular quantization level of the table. Data discrepancy depends upon the resolution of the table employed to compress and decompress the data. Known fixed-length quantization tables exhibit a number of levels dictated by the number of bits used to represent the output codewords, and all codewords are represented by the same number of bits within a given table. For example, a table with three-bit output codewords has eight levels (23), and a four-bit table has sixteen levels (24). The average resolution of the table, and generally of each level, is the domain of the table divided by the number of levels in the table.
In quantization tables, output quantized data are represented by multiple-bit words, or symbols. It is herein recognized that for certain types of data, quantization tables employing symbols with less than the predominant number of bits per symbol for at least one quantization level can significantly reduce bandwidth and memory requirements. In accordance with the principles of the present invention, compression and decompression tables have N levels associated with respective symbols of predominantly M bits, except that at least one level receiving values at a known general rate is compressed with a symbol having less than M bits.